Particle accelerator



June 9, 1959 TlHlRo oHKAwA 2,890,348

PARTICLE ACCELERATDR Filed July 8, 1957 INVENTOR. I'fu'fo Okkazaa in producing high-energy particles.

United States Patent O States of America as represented by the United States Atomic Energy Commission 'Application July '8, 1957, 'Serial No. 670,622

7 Claims. (Cl. Z50-84) Thepresent invention relates generally to particle accelerators, and also to devices lfor producing a physical reaction by collision.

' Circular accelerators have proved to be a useful tool In such machines, charged particles, such as protons and deuterons, are introduced into an orbit and revolved about the axis of the accelerator gaining energy with each revolution. The radius of the orbit of the charged particle is a function of the energy of the particle and the magnetic field traversing the orbit. As a result, the orbit radius of the particle increases with increased energy, the magnetic eld being held constant, or the radius of the orbit may be held constant by increasing the magnetic eld as the energy of the charged particle increases.

The fixed iield alternating gradient accelerator, abbreviated FFAG, envisages a time independent magnetic field which has symmetry about a medium plane, increases as a power of the radius, and varies periodically with azimuth. Particles of a wide range of energies have stable orbits in a medium of this configuration and may be accelerated.

It is one of the objects of the present invention to provide a xed field alternating gradient circular accelerator for the simultaneous acceleration of two particle beams in opposite directions.

In such a .two beam accelerator, the periodic azimuthal variation of magnetic lield can be expressed as an odd function of the azimuthal variable and has an average of zero. Particles are at larger radii in magnets of positive curvature (where their trajectories bend in the correct direction to circulate about the center of the accelerator) and because of the increase of eld with radius are in a eld of higher magnitude. In magnets of negative curvature, particles are at smaller radii and are therefore in a iield of smaller magnitude. The average eld along the particle orbit is thus positive and particles circulate about fthe accelerator.

It is desirable to collide two beams of high-energy particles to produce certain physical reactions. It is therefore a further object of the present invention to provide an accelerator for simultaneously producing two high-energy beams of particles, and producing the collfision between these beams after the beams have attained a suitable energy.

These and additional objects of the present invention will become readily apparent to those skilled in the art from a further reading of this disclosure, particularly when viewed in the light of the drawings, in which:

Figure 1 is a plan view of a circular particle accelerator constructed according tot the teachings of the present invention;

Figure 2 is a sectional View taken along the line 2 2 of Figure 1; `and Figure 3 is a sectional view taken along the line 3--3 of `Figure 1, this figure also illustrating the accelerator combined with a spiral spectrometer.

ICC

To laccommodate twovbeams which proceed in opposite directions about a circular accelerator, the accelerator must be a scaling accelerator, that is, all orbits must be magniiied replicas of the :lowest energy orbit. In such an accelerator, the magnetic field averaged in the azimuthal direction must be zero. Also, in order to obtain closed orbits, the magnetic eld averaged along the orbits has nite values of opposite signs depending upon the direction of the beam. These conditions are met in the circular accelerator described Iin Ithe patent application of Lee C. Teng entitled Particle Accelerator tiled January 3l, 1957, Serial No. 637,595, and reference is hereby made thereto. The principles of fixed eld alternating gradient accelerators `are also discussed by K. R. Symon et al., Physical Review 103, 1837 (1956).

As indicated in the figures, a circular accelerator constructed `according to the teachings of the present invention has a vacuum tight orbit chamber 54 which may be in the form of a at, hollow cylinder. The chamber 54 `is highly evacuated. The accelerator is also provided with a magnetic iield of iixed intensity which traverses the orbit chamber 54 in a direction normal to lthe plane of the particle orbits. The magnetic field alternates in direction along approximately equally spaced radial sectors, and increases from the innermost orbit of the accelerator to the outermost orbit. This magnetic eld is maintained by a plurality of electromagnets 60 which are equally spaced about the periphery Iof the orbit chamber 54. Each of the electromagnets 60 has a C-shaped yoke 62 which curves longitudinally to conform to the curvature of the orbit chamber 54. Each yoke 62 abuts the iiat surfaces of the orbit chamber 54, and is spaced from `the cylindrical surface of the orbit chamber to form a channel 64 for an electrical coil 66. The coil 66- extends through the channel 64, about the ends of the yoke 62 and across the Side of the yoke 62 opposite the channel 64, as illustrated inuligure 2. Adjacent magnets are polarized in opposite directions to produce scalloped particle orbits.

A cavity resonator 68 is disposed between a pair of adjacent electromagnets 60 to accelerate the particles revolving in the orbits within the chamber 54. The resonator 68 is in the form of an electrically conducting plate and extends radially through the wall of the orbit chamber 54 parallel to the axis thereof. The resonator 68 is provided with an aperture 70 which permits the particles within the orbit chamber 54 to pass through the resonator. The resonator 68 is connected to a pulse source 72 which places a potential on the resonator to accelerate the particles. An interruptor 73, which may be as elemental as a switch, is connected to source 72 as shown in Fig. l; the function of the interruptor will be explained later.

Two injectors 74 and 76 are employed to inject bunches of particles into the orbit chamber 54. Injectors 74 and 76 may be linear accelerators, such as Van de Graai accelerators; the injectors may also be of other types as disclosed by M. Stanley Livingston in High Energy Accelerators, Interscience Publishers, New York, 1954. Injector 74 is connected with the orbit chamber through a tubular dellecting means 78, and the injector 76 is connected into the orbit chamber 54 by a similar deflecting means 80. The particles from the injector 74 are introduced into the orbit chamber 54 adjacent to the inner edge of one of the magnets 60a, and the particles are traveling in a counterclockwise direction relative to the axis of the orbit chamber 54. In like manner the particles from the injector 76 are introduced into the orbit chamber adjacent to the inner edge of the magnet 60b and are traveling in a clockwise direction.

The particle injectors 74 and 76 inject short bunches of particles into the orbit chamber` 54 in response to electrical pulses from the source 72. A synchronizing network 82 is connected to the source 72 and the pulse injector 74, and a phase-shift network 84 is connected between the particle injector 74 and the particle injector 76.

Figure 3 illustrates the orbits of the two particle beams, designated 85 and 86. Since the accelerator is a scaling machine, each of the orbits 85 and 86 is a mere enlargement of the innermost orbit for that particular beam. Further, it is to be noted that the beam 85 is disposed radially inwardly from the beam 86 in magnets directed to curve the beam 85 radially outwardly and vice-versa. 'Ihe injector 74 is positioned so that it injects bunches of charged particles into the magnetic field of a field sector directed opposite to the magnetic field of the field sector into which particles are injected from the injector 76. Stated another way, there is an even number of field sectors between injectors 74 and 76; in Fig. l, the injectors are separated by two field sectors. As a result, the initial deflection of the charged particles from the injector 74 is the same as the initial deflection of the particles from the injector 76, and the beams 85 and 86 are deflected in opposite directions in each radial sector of the magnetic field, the radial sector of the magnetic field being the region in which the magnetic field flows in a direction opposite to the direction of the adjacent sectors.

It Iwill be noted that the two beams 85 and 86, which are of the same energy, pass through common points located approximately at the interface between the adjacent sectors of the magnetic field. Collision would occur at these points if the beams 85 and 86 were of the same betatron oscillation phase, even though the particles in each beam are bunched in short bunches relative to the wavelength of the betatron oscillations. The phase difference between the two beams 85 and 86 is obtained as a result of the time of injection from the two injectors 74 and 76 relative to the frequency of the source 72. The source 72 produces an electrical signal approximately in the form of a sine wave. The betatron oscillation phase of one beam is such that a particle bunch of the beam is disposed within the resonant cavity 68 during the period that the accelerating pulse is rising, while the phase of the other beam is such that a particle bunch of this beam is disposed within the resonant cavity 68 during the period that the accelerating pulse is declining. The betatron oscillation phase angle, qs, between the null potential point and the point at which the first beam is disposed Iwithin the resonant cavity is known as the stable-fixed phase of synchrotron oscillation for this beam, and the phase difference between the betatron oscillations of the two beams 85 and 86 is (11-2s). In this manner, the magnitudes of the accelenating potential for both beams are approximately equal and both beams receive approximately the same acceleration.

As long as the radio-frequency excitation from the source 72 is present, the particle bunches in each of the beams 85 and 86 cannot collide, and the two beams are simultaneously accelerated. However, if the radio-frequency excitation is interrupted, as by interrupter 73, the particles begin to spread around the circumference and collide with each other. Therefore, particles may be made to collide at any energy up to maximum energy of the machine by interrupting the radio-frequency source, and periodic collisions of the same average energy particles can be obtained by interrupting the source 72 periodically at the proper time interval.

Collision of the two beams may also be achieved by adding a small amount of average magnetic field. In this case, the beam in one direction moves radially outwardly, while the beam in the opposite direction moves in, so that the beams can be made to collide atany point along their orbits. The frequencies of betatron oscillation are, in general, different in the two directions in this case.

It is to be noted from Figure 3, that the points of collision occur approximately at the interface between .the

magnetic sectors, and at these points both of the beams are either moving radially outward or radially inward. Therefore, the resulting particles, or products, following the collision will move either radially outward or radially inward, as indicated by the arrows in Figure 3. Such particles or products of the collision may be subatomic, i.e. electrons, protons, neutrons, positrons, mesons, neutrinos, alpha particles, gamma rays, or light photons; or they may be atomic or molecular in nature, depending on the nature, energy, density, angle-of-collision, and other characteristics of the colliding particles, and the presence and essence of target materials other than the colliding particles at the place of collision. Since the illustrated machine contains six magnetic sectors, three beams of particles resulting from collisions will move inwardly, while three beams will move outwardly. The beams of particles moving toward the axis of orbit chamber 54 will produce a high-density particle region, and under some circumstances, this region may sustain physical reactions. In general, very high vacuums are required for physical reactions, of the order of 10-10 atmospheres, so that particles from the beams will not scatter from the residual gas and be lost.

The products of collisions which move radially outwardly may also be utilized to perform bombardments. Figure 3 illustrates a spiral spectrometer disposed about the accelerator to utilize these outwardly moving beams of particles. An evacuated circular chamber 87 is coaxially disposed about the cylindrical orbit chamber 54. The orbit chamber 54 is provided with apertures 88 confronting alternate interfaces of the magnetic sectors of the accelerator, these interfaces being the interfaces at which the beams and 86 are moving radially outwardly. As a result, the outwardly moving particles pass through the apertures 88, and through sleeves 90 connecting the apertures 88 to the annular chamber 87. Targets 92 and 94 are disposed in the annular chamber 87 adjacent to each of the magnets 60 on opposite sides of the apertures 88, and a radiation shield 96 is disposed between the magnets 6i) and the annular chamber 87. A concentric ring magnet 98 having poles on opposite sides of the chamber 87 provides a magnetic field normal to the plane of Figure 3, and causes the outwardly moving particles to separate into positive and negative particles .and bombard the targets 92 and 94. Spiral spectrometers are well-known in the art, having been described by G. Miyamoto in the Proceedings of the Physical Society of Japan 42, 676 (1942), and 43, 557 (1943). ln general, a spiral spectrometer functions to sort and detect particles of varying mass, charge, and energy by virtue of an elaborate magnetic field which exerts a .dif-ferent influence on particles depending on the particular masses, charges, and energies thereof.

While the magnetic field in the accelerator is formed by adjacent magnetic field sectors of opposite direction, it is achieved by spaced magnets 60, the portions of the fields between the magnets having relatively small magnetic flux. The magnitude of the scalloped motion, and hence the deflection angle per sector, may be increased by increasing the space between adjacent magnets. This is achieved, however, with an increase in the circumference of the accelerator under a given set of conditions, and there is an optimum condition to achieve the smallest possible circumference factor keeping fixed the working points and the orbits of the two beams v85 and 86.

Also, if the axial focusing is not sufficient, a small amount of edge-.focusing can be added by shaping the edges of the magnets spirally, rather than radially.

YIt is to be understood, that an accelerator constructed according to the present invention may be used to produce two beams for non-related purposes as well as for interaction between the beams. For example, the two beams may by conventional means be extracted from the accelerator for the bombardment of two unrelated targets.

Those skilled in the art fwill readily devise many de# vices and modications to the disclosed accelerator within the spirit of the present invention. It is, therefore, intended that the scope of 4the present invention be not limited by the foregoing disclosure, but rather by the appended claims.

What is claimed is:

1. A iixed field alternating gradient accelerator comprising a vacuum chamber providing a plane for particle orbits, means providing a fixed intensity magnetic eld generally normal to the particle orbit plane, said eld alternating in direction along approximately equally spaced radial sectors and increasing from the innermost orbit to the outermost orbit, a cavity resonator disposed in the particle orbits, a radio-frequency source connected to the resonator, a rst means synchronized with the radio-frequency source to inject electrically charged particles in bunches into the innermost orbit of the accelerator directed for clockwise rotation about the orbit axis to form a first particle beam having betatron oscillations as it revolves about its orbit, and a second means synchronized with the radio-frequency source to inject electrically charged particles in bunches into the innermost orbit of the accelerator directed for counter clockwise rotation to form a second particle beam having betatron oscillations as it revolves about its orbit, the betatron oscillations of said second beam diiering in phase from the rst beam.

2. A xed eld alternating gradient accelerator comprising a vacuum chamber providing a plane for particle orbits, means providing a xed intensity magnetic eld generally normal to the particle orbit plane, said field alternating in direction along approximately equally spaced radial sectors and increasing from the innermost orbit to the outermost orbit, a cavity resonator disposed in the particle orbits, a radio-frequency source connected to the resonator, a rst means synchronized with the radio-frequency source to inject electrically charged particles in bunches into the innermost orbit of the accelerator directed for clockwise rotation about the orbit axis to form a lirst particle beam having betatron oscillations as it revolves about its orbit, and a second means synchronized with the radio-frequency source to inject particles having the same electrical charge in bunches into the innermost orbit of the accelerator directed for counterclockwise rotation to forma second particle beam having betatron oscillations as it revolves about its orbit, the betatron oscillations of said second beam diiering in phase from the rst beam by 180 degrees -2 where ps is the stable fixed phase of synchrotron oscillations.

3. A xed field alternating gradient accelerator comprising the elements of claim 1 in combination with an interrupter connected between the radio-frequency source and the resonant cavity.

4. A device for bombarding objects with particles comprising, in combination, a spiral spectrometer disposed coaxially about the accelerator of claim 3.

5. A xed field alternating gradient accelerator comprising a vacuum chamber providing a plane for particle orbits, means providing a xed intensity magnetic field generally normal to the particle orbit plane, said eld alternating in direction along approximately equally spaced radial sectors and increasing from the innermost orbit to the outermost orbit, a cavity resonator disposed in the particle orbits, a radio-frequency source connected to the resonator, a rst means synchronized with the radio-frequency source to inject electrically charged particles in bunches into one of the radial tlux sectors and into the innermost orbit of the accelerator directed for clockwise rotation about the orbit axis to `form a iirst particle beam having betatron oscillations as it revolves about its orbit, 'and a second means synchronized with the radio-frequency source to inject particles of opposite electrical charge in bunches into a radial ux sector of the same direction and into the innermost orbit of the accelerator directed for counter clockwise rotation about the orbit axis to form a second particle beam, the betatron oscillations of said second beam having betatron oscillations as it revolves about its orbit differing in phase from the first beam by degrees ,2b where s is the stable iixed phase of synchrotron oscillations.

6. A device for bombarding nuclear particles comprising: an accelerator having a vacuum chamber providing a plane for particle orbits, means providing a fixed intensity magnetic iield generally normal to the particle orbit plane, said iield alternating in direction along approximately equally spaced radial sectors and increasing from the innermost orbit to the outermost orbit, a cavity resonator disposed in the particle orbits, a radio-frequency source connected to the resonator, a rst means synchronized with the radio-frequency source to inject electrically charged particles in bunches into one of the radial ilux sectors and into the innermost orbit of the accelerator directed for clockwise rotation about the orbit axis to form a rst particle beam having betatron oscillations as it revolves about its orbit, and a second means synchronized with the radio-frequency source to inject particles having the same electrical charge in bunches into a radial flux sector of the opposite direction into the innermost orbit of the accelerator directed for counter clockwise rotation to form a second particle beam having betatron oscillations as it revolves about its orbit, the betatron oscillations of said second beam differing in phase from the rst beam by 180 degrees -2s, where qbs is the stable fixed phase of synchrotron oscillations; and a spiral spectrometer having an annular evacuated housing disposed coaxially about the vacuum chamber, said housing being connected to the vacuum chamber by channels disposed on the radial interfaces between adjacent radial magnetic sectors, and means to maintain a magnetic ux through the housing parallel to the axis of the accelerator.

7. A device for bombarding particles comprising the elements of claim 6 wherein radiation shields are disposed between the housing of the spiral spectrometer and the accelerator.

References Cited in the file of this patent UNITED STATES PATENTS 1,645,304 Slepian Oct. 1l, 1927 2,538,718 Wideroe Jan. 16, 1951 2,599,188 Livingston June 3, 1952 2,790,902. Wright Apr. 30, 1957 

